Refactored reduction kernels

[Micky774]

Description

Merged similarly-structured kernels into a single templated kernel w/ re-usable dispatch, as well as improvements to logging.

Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

ck_fused_attn_utils.hpp

• Add to_f32<T>() / from_f32<T>() device helpers to centralize bf16↔float casting.

ck_fused_attn_bwd.cpp

• Merge 4 dk/dv kernels (dk_dv_reduce, dk_or_dv_reduce, *_thd) into one dkv_reduce<DataType, GroupMode, ReduceBoth>.
• Merge 3 dbias kernels (dbias_reduce_{11ss,1hss,b1ss}) into one dbias_reduce<DataType, ReduceB, ReduceH>.
• Add log_dkv_reduce / launch_dkv_reduce / launch_dbias_reduce helpers; collapse the verbose per-launch logging + dispatch blocks in ck_attn_bwd.
• Drop unused s_kv kernel param.

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes